1. Field of the Invention
The invention relates to a short arc lamp as is used for a light source for purposes of UV exposure in production of LSI or for similar purposes.
2. Description of Related Art
In a short arc lamp, within a spherical or oval arc tube made of fused silica glass, an anode and cathode are disposed opposite one another with a gap therebetween. A side tube is connected to each of opposite ends of the arc tube and in which upholding parts of the cathode and anode are sealed. To hold the cathode and anode upholding parts securely, a negative pressure state is caused in the arc tube and the ends of the side tubes are heated so that the diameter of the side tube is reduced and shrunken areas are formed. To directly hold the upholding parts of the electrodes of the cathode and anode in the shrunken regions of the side tube, it is necessary to draw the side regions very strongly. The outer peripheral surfaces of the upholding parts of the cathode and anode are therefore wound with molybdenum foils with a thickness from roughly 15 to 30 microns. The wound locations are inserted and held in fused silica glass cylindrical retaining bodies, and the shrunken regions of the side tubes are welded to the cylindrical retaining bodies. In this way, the cylindrical retaining bodies are integrally joined with the shrunken areas of the side tube and attached. Furthermore, the advantage resulting from welding the shrunken regions of the side tube and the cylindrical retaining bodies to one another in an integral arrangement is that the thickness of these locations becomes greater and in this way strength is increased. The reason for winding of the upholding parts of the electrodes with molybdenum foil is to absorb the expansion of the upholding parts of the electrodes with increases in temperature which occur during lamp operation, and to prevent a high load from being exerted on the cylindrical retaining bodies.
During the drawing process, it is necessary to temporarily attach the cylindrical retaining bodies to the upholding parts of the electrodes to prevent the cylindrical retaining bodies from moving along the upholding parts of the electrodes. Since, on the sides of the cylindrical retaining bodies which are opposite the electrodes, there are conductive foils for purposes of power supply, the cylindrical retaining bodies can be temporarily attached using these foils. This prevents the cylindrical retaining bodies from moving in away from the electrodes.
Therefore, temporary attachment which is used to prevent movement of the cylindrical retaining bodies towards the electrode side is needed. Conventionally, therefore, the following was done, as shown in FIG. 4:
One end of a temporarily attached molybdenum plate 28 with a thickness of roughly 0.1 mm, and which is made into the form of a strip, is welded to the upholding parts 20 of the electrodes.
The outer peripheral surface of the upholding parts 20 of the electrodes which surround this welded site, i.e. essentially half the temporarily attached plate 28 in the longitudinal direction, is wound with a molybdenum foil 27, and is inserted in a cylindrical retaining body 30 in a state in which the remaining half of the temporarily attached plate 28 projects from the retaining body 30.
The temporarily attached plate 28 which projects from one end of the cylindrical retaining body 30 is curled and brought into contact with a face of the cylindrical retaining body 30, as is illustrated in FIG. 5. In this way, the cylindrical body 30 is temporarily attached in the upholding parts 20 of the electrodes.
But, this temporary attachment had the following disadvantages:
Since the end of the temporarily attached plate which is made into the form of a strip is welded to the upholding parts of the electrode, and furthermore, the part of the temporarily attached plate which projects from an end of the cylindrical retaining body is curled, greater production cost is necessary.
In the winding of the temporarily attached plate with molybdenum foil, there are cases in which the thin molybdenum foil is cut by the edge of the temporarily attached plate. Furthermore, in bending the temporarily attached plate into the form of a curl, there were cases in which the molybdenum foil was cut by the temporarily attached plate moving upward.
Since the curled part of the temporarily attached plate bent is in contact with the face of the cylindrical retaining body at only one point, the welded site of the temporarily attached plate is damaged by the weight of the cylindrical retaining body, if the side tube is shrunk while the arc tube is being turned. Therefore, there were cases in which the cylindrical retaining body moved, i.e. temporary attachment was only incompletely obtained.
During lamp operation, there were cases in which, immediately after turning on and before starting the discharge between the electrodes, an anomalous discharge occurred proceeding from edges of the temporarily attached plates. While the starting point of the discharge moves from the edges of the temporarily attached plates to the electrode tips until a normal discharge takes place, the temporarily attached plates begin to partially melt. A molten mass of molybdenum is deposited on the boundary regions between the cylindrical retaining bodies and the shrunken regions of the side tube. Therefore, there were cases in which cracks formed in these regions of the arc tube and in the shrunken regions. Furthermore, there were also cases in which the lamp finally broke when these cracks grew.
Therefore, a primary object of the invention is to devise a short arc lamp in which cylindrical retaining bodies can be easily and reliably attached in upholding parts of the electrodes on a temporary basis without damage to the molybdenum foils with which the upholding parts of the electrodes are wound, and in which crack formation in the boundary regions between the arc tube and the shrunken regions can be prevented, even if immediately after starting of lamp operation an anomalous discharge forms.
According to the invention, in a short arc lamp in which the arc tube contains a cathode and an anode disposed opposite one another, and in which upholding parts of the cathode and anode are inserted and held in cylindrical retaining bodies, and in which the cylindrical retaining bodies are supported in the shrunken regions of side tubes connected to the arc tube, this object is achieved by the upholding parts of the electrodes being wound with coil components which border the electrode-side faces of the cylindrical retaining bodies and by the cylindrical retaining bodies being temporarily attached by these coil components.
According to another aspect of the invention, the indicated object is achieved by grooves in which the coil components fit being formed in the upholding parts of the electrodes. This measure prevents the coil components from moving. Therefore, the effect of the temporary attachment of the cylindrical retaining bodies can be ensured even more.
When coil components are used as the components for temporary attachment, there is a great probability that the starting point of the anomalous discharge will be located on the sharp ends of the flexible coil leads immediately after starting. Such a problem is solved, however, according to the invention, the coil being wound in two stages and by its having been formed by bending the middle region of the flexible coil lead and by two-layer winding, in which the two ends of the flexible coil lead are located on the same side, and by the ends of the flexible coil lead being located on the electrode sides.
By this measure, the two ends of the flexible coil lead, i.e. the starting point of the anomalous discharge, are located in positions away from the cylindrical retaining bodies, i.e. in positions which are away from the border regions between the cylindrical retaining bodies and the shrunken regions of the side tube.
This prevents the molten mass from being deposited on the border regions between the cylindrical retaining bodies and the shrunken regions of the side tube, even if the ends of the flexible coil lead begin to melt. Therefore, crack formation can be prevented.
However, the object can also be achieved in accordance with the invention by the coil components being formed from tungsten which contains emitters. This measure can suppress vaporization of the tungsten of the coil components and prevent blackening of the arc tube.
In the following the invention is further described using several embodiments shown in the drawings.